1. Field of the Invention
The present invention relates to a machine tool, and more particularly, to a thermal displacement correction device for the machine tool.
2. Description of the Related Art
Since a feed screw and a spindle of a machine tool are driven by a motor, they are expanded so that their mechanical positions change due to heating of the motor, frictional heat generated by the rotation of a bearing, and frictional heat from a contact portion between a ball screw and a ball nut of the feed screw. Thus, the relative positions of a workpiece to be positioned and a tool are shifted. The change of the mechanical positions due to heating hinders high-accuracy machining.
The displacement of the mechanical positions due to heating can be eliminated by using a technique in which a command position is corrected based on a displacement or temperature detected by a displacement or temperature sensor or a structure configured to apply an initial tension to the feed screw without being influenced by thermal expansion.
The technique for correcting the command position can be roughly classified into two categories.
In one of these categories, an actual thermal displacement is periodically detected for correction by using a touch probe, for example. In the other category, correction is made in real time by predicting a thermal displacement amount from a value detected by the sensor and the operating state of the machine tool. In these cases, accuracy and time are in a trade-off relationship. Accordingly, a method is proposed in which the overall accuracy of correction is improved by suitably detecting and modifying an actual thermal displacement amount, although the thermal displacement amount is basically predicted and corrected in real time.
According to Japanese Patent Application Laid-Open No. 2002-18677, a thermal displacement amount attributable to axial movement of a feed shaft is estimated for each of a finite number of sections into which the entire stroke of the feed shaft is divided. The distribution of thermal displacements in individual positions of a feed screw can be estimated by adding up the thermal displacement amounts for the sections from a reference position to a correction position. Thus, accurate correction can be made without regard to the position of the feed shaft. In addition, the thermal displacement can be accurately corrected in consideration of a thermal displacement due to heating of a spindle or spindle motor. Furthermore, more accurate correction is made by modifying a heating factor in a thermal displacement amount calculation formula, based on a deviation (correction error) between an estimated thermal displacement amount (correction amount) and an actual mechanical position.
According to Japanese Patent Application Laid-Open No. 2012-101330, an initial position is stored in advance by a position detection sensor and a position in which the next signal is output is detected as an actual position. An error correction factor is calculated based on the difference between the initial position and the detected actual position, whereby a thermal displacement amount is modified. Correction is made also in consideration of a change in ambient temperature or other thermal displacements irrelevant to machine operation.
According to Japanese Patent Application Laid-Open No. 11-90779, a thermal displacement amount is measured after the drive of a machine tool is started, and a calculated value of a thermal displacement correction amount is modified based on the measured value. Specifically, a difference obtained by subtracting the calculated value from the measured value or the ratio between the measured and calculated values is calculated as a correction value, and the thermal displacement amount is modified by adding the correction value to the calculated value or multiplying the calculated value by the correction value. An accurate thermal displacement amount can be calculated depending on various properties of the machine tool and the operating environment by modifying the calculated value of the thermal displacement amount based on the measured value.
According to Japanese Patent Applications Laid-Open Nos. 2002-18677, 2012-101330, and 11-90779, if chips are caught or a coolant splashes so that the result of detection by a position detection sensor is improper, a factor or thermal displacement correction amount may possibly be modified by mistake so that the result of correction becomes worse.
Disclosed in Japanese Patent Application Laid-Open No. 2002-18677, moreover, is the method for modifying the heating factor in the thermal displacement amount calculation formula based on the correction error. Since the calculation formula includes other factors or coefficients (a heat radiation coefficient and a heat conduction coefficient for calculating heat conduction from each adjacent section), the accuracy of the correction sometimes cannot be further improved by only modifying the heating factor.
In the technique disclosed in Japanese Patent Application Laid-Open No. 11-90779, moreover, the measured value of the thermal displacement amount is calculated by comparing a calculated drive amount with a drive amount obtained before a tool contacts a detector. If the calculated drive amount is different from the latter due to deformation of a column or the like, therefore, a problem remains that the thermal displacement cannot be satisfactorily corrected.